Messages app

I'm posting this in the Beta forum since this particular issue has occurred twice since being in the beta (though it might be a coincidence).

Over the past week, I've had two people that I've messaged (Google messages) where they both replied, but I never got the message. (I've only recently moved to Google messages as a troubleshooting attempt from when I was with TMO. I'm now with AT&T.)

In both instances, the sender was using iPhone. I don't know their carriers, but I do know that both did, in fact reply. I've just switched back to Samsung Messages, and confirmed with one party that I could receive a return text. I'm curious if anyone has any thoughts or speculation on this. It could be an RCS issue with Google Messages, an iPhone issue with Google Messages, etc. I'm just not clear.